1. Field of the Invention
The present invention relates to an automatic white balance circuit which is employed for a color video camera, a color electronic camera or the like.
2. Description of the Background Art
In the image of an object taken by a color video camera, for example, the white balance is varied with ratios between color components of light which is applied to the object. Therefore, the white balance must be adjusted following change in the ratios between such color components of light which is emitted from a light source, in order to obtain a color image of high quality in a color video camera. To this end, an automatic white balance circuit is generally employed together with an image pickup element, to measure the ratios between color components of light which is applied to the object of the image pickup element, for automatically adjusting the white balance on the basis of the result of such measurement.
FIG. 1 is a block diagram showing partial structure of a conventional color video camera having such an automatic white balance circuit. Light from a light source 1 is reflected by an object 2, and supplied to an image pickup element 4 through an optical lens 3. The image pickup element 4 converts the received light into electric signal and supplies the same to a color signal separation circuit 5. The color signal separation circuit 5 separates the electric signal into three electric color signals and supplies the same to a white balance adjusting circuit 15. The white balance adjusting circuit 15 amplifies the respective color signals in response to a white balance control voltage F as hereinafter described, and outputs the same.
The light from the light source 1 is also supplied to color filters 7 and 8. The color filters 7 and 8 have different transmission characteristics, so that different specific color components pass through the color filters 7 and 8, respectively. Light receiving elements 9 and 10 receive the color components and convert the same into currents I.sub.1 and I.sub.2, respectively. A current-to-voltage conversion circuit (hereinafter referred to as "I-V conversion circuit") 11 converts the current I.sub.1 into a logarithmically compressed voltage of A+BlogI.sub.1, where A and B represent constants. Another I-V conversion circuit 12 converts the current I.sub.2 into a logarithmically compressed voltage of C+BlogI.sub.2, where C represents a constant. A subtraction circuit 13 receives the voltages output from the I-V conversion circuits 11 and 12, to output a difference voltage E of D+Blog(I.sub.1 /I.sub.2), where D represents a constant. A rectifying circuit 14 integrates the difference voltage E from the subtraction circuit 13. The output of the rectifying circuit 14 is supplied to the white balance adjusting circuit 15 as the white balance control voltage F. The white balance adjusting circuit 15 changes amplification degrees of the respective color signals in response to the white balance control voltage F, to adjust the ratios between the color signals. Thus, the white balance adjusting circuit 15 outputs the color signals, which are subjected to white balance adjustment.
In the conventional color video camera having the aforementioned structure, the following problems are caused when the light source 1 is formed by a flashing one such as a fluorescent lamp, which flashes too instantaneous for visual recognition: As shown in FIG. 3, output voltages X and Y of the I-V conversion circuits 11 and 12 have waveforms which are synchronous to flashing of the light source 1, as shown in FIG. 2. The output voltage E of the subtraction circuit 13 also forms waveshape corresponding to the flashing of the light source 1, as shown in FIG. 2, since each color component of light varies its fading speed when the light source 1 goes off. The aforementioned white balance adjustment is therefore performed by a white balance control voltage F which is integrated signals including an output voltage E of the subtraction circuit 13 when the light source 1 goes off. Meanwhile, when the flashing of the light source 1 is too instantaneous for human eyes, human eyes receive information given when the light source 1 is on, and image pickup element 4 outputs the signal based on information given when the light source 1 is on. However, aforementioned white balance adjustment is still preformed in such a case of the flashing light source described above, whereby the white balance gets rather out of order.